Assuming the Second World War begins and ends without any sort of intervention by the United States (the threat of Germany and Japan either peters out or is crushed by the other powers), then what would be the changes to American industry,..
WWII caused a massive reconstriuction & expansion of US industry. ie: The aicraft industry went from a 1938 capacity of under 10,000 aircraft engines per year to over 300,000 per year at the peak in 1944. In the case of the railwaysclose to half the tracks & support were rebuilt during the war, the balance had a complete mainitnace & improvement program. Obsolete & worthless lines were removed & material salvaged.
It was the same across the board, but more important that the physical plant was the conversion of the Depression era labor. Millions of unemployed and unskilled became semi skilled and skilled workers. Millions of others still working with obsolete skills were brought up to the latests skill levels. At the end of 1944 the US had recovered a ultra modern and highly productive labor force. This included production supervisors and higher management who who had learned a high standard of efficiency in daily operations.
The US controled in 1945 the balance of the usable capitol of the globe.
The US Army remains a small cadre of officers & NCOs educating themselves for a furure war. The few development projects under way would continue & as economic recovery occured some of the projects of the 1920s would be revived. A few experimental formations would receive new material, but the bulk of the Army and National guard would remain equipped for a war of 1919.
The navy would continue much of the development of OTL, but in smaller numbers. ie: the BB classes planned in the 1930s would be built but one or two fewer in each class. Just two or three of the Essex class carriers would be built to replace the oldest carriers, and their construction dragged out. they might not even be laid down until 1944. The same for all the other ship types through the 1940s.
in the aftermath of the 'triumph' of isolationism?
Isolationism of the interwar is frequently misunderstood. Its effect was to with hold US support for the former Entente nations vs German resuergency & Facism. Particularly the French democracy. However US intervention in Europe did occur. Things like midwifing affordable loans to Germany in the 1920s & the Nuetrality Acts influenced European politics as surely as written treaties.
Beyond that isolationists seem to have not applied to Asia. Through the interwar years the US kept a naval force permanetly in Chinese territory, & in 1927-28 sent a mixed expeditionary force of Navy, Marines, and Army. The naval base on Luzon PI was slowly improved, and the Army forces in PI improved. Oahu had its old Pearl harbor coaling station turned into a major naval base. On the political side the US was active in Chinese and Japanese international and internal affairs. That slacked off somewhat in the 1930s, but that was as much to do with Depression economics & specific politics of the Roosevelt administation.
Without WWII the US remains economically crippled into the 1940s & perhaps beyond. The other industrial nations have their industry and labor forces intact, their cities not burned to the ground. The US likely remains a "Sleeping Giant" into the 1950s and beyond.